1. Technical Field
It relates to a method for manufacturing a metal electrode having a transition metal oxide coating layer and a metal electrode manufactured thereby.
2. Description of the Related Art
Higher value-added businesses which collect and use various and useful information in real time by employing IT equipments receive attentions and stable energy supply for securing reliability of such systems becomes an important factor in the information-oriented society. These IT equipments and electrical devices include electric circuit boards and each circuit board has a capacitor which stores an electric charge and releases it when required and thus stabilizes energy flow in the circuit. This capacitor has a very short charge/discharge time, a long lifetime and a high power density but generally a very low energy density. This disadvantage of low energy density causes many limitations on its use as an energy storage device.
However, electrochemical capacitors, supercapacitors or ultracapacitors, which have started to be commercialized in Japan, Russia, USA, etc. since 1995, are under development in all countries of the world to provide higher energy density as next generation energy storage devices along with secondary batteries.
A supercapacitor can be broadly classified into 3 categories depending on the electrode and the mechanism: (1) an electric double layer capacitor (EDLC) which employs activated carbon as an electrode and is based on an electric double layer electric charge absorption mechanism; (2) a metal oxide electrode pseudocapacitor (or redox capacitor) which employs a transition metal oxide and a conductive polymer as an electrode material and is based on a pseudo-capacitance mechanism; and (3) a hybrid capacitor which combines the features of both electrochemical and electrolytic capacitors. Among them, the EDLC-type supercapacitor using activated carbons is currently used the most.
The supercapacitor is composed of an electrode, an electrolyte, a current collector, and a separator and is based on the electrochemical mechanism which stores energy through absorption of electrolyte ions on the electrode surface by migrating along with the electric field when voltages are applied on the both ends of a unit cell electrode. Since the specific capacitance is proportional to the specific surface area, the supercapacitor improves energy (storage) density through the use of an activated carbon electrode, which is a porous material. An electrode is manufactured by preparing slurry including a carbon electrode material, a carbon conductive material and a polymer binder and coating the slurry on a current collector. Here, it is important to improve adhesiveness to the current collector and reduce contact resistance at the same time and further lower internal contact resistance between activated carbons by changing a ratio or kind of the binder, the conductive material and the electrode material.
When a pseudocapacitor using a metal oxide electrode material is used, the transition metal oxide exhibits higher capacity and higher power density compared to activated carbons. Recently, it has been reported that amorphous hydrate electrodes exhibit much higher specific capacitance.
Since the electric capacitance is proportional to the specific surface area, it is needed to use an electrode material having high specific surface which is the most essential factor to improve a capacitance of a supercapacitor. In addition, it is needed to have high conductivity, electrochemical inactivity, easy forming and processability and the like. Carbon materials which satisfy such properties have been widely used the most. Examples of such porous carbon materials may include activated carbon, activated carbon fiber, amorphous carbon, carbon aerogel or carbon composite, carbon nanotube and the like. However, even though such activated carbons have high specific surface area, effective pores of the activated carbon are only about 20% because most pores are micropores of which diameter is about 20 nm or less and which cannot do much for an electrode role. Since the electrode is prepared from slurry which is formed by mixing a binder, a carbon conducting material and a solvent, etc. an actual effective contact area between an electrode and an electrolyte is decreased. There are further drawbacks such as uneven electric capacitance and contact resistance between an electrode and a current collector.
KR patent application no. 2003-0099761 discloses a method for manufacturing an electrode for supercapacitors by using a metal oxide, instead of using a binder, to increase effective contact area but it still has drawbacks such as low conductivity, contact resistance and high manufacturing cost, etc.